There is a perceived threat of exposure of a sizable human population to radiation through terrorist action. Currently, there is only one chemical, amifostine, approved as a radioprotector, which is rather toxic at its therapeutic dose. There is an urgent need for more such chemicals for the mitigation and treatment of radiation injury. Genetic instability, including DNA deletions are involved in cell death, cellular dysfunction as well as long-term consequences such as birth defects and maybe other degenerative diseases. Radiation causes DNA strand breaks resulting in genetic instability and DNA deletions. We have previously shown that DNA deletions are inducible by ionizing radiation in yeast, human cells as well as in vivo in mice. In the previous budget period we have developed a high tliroughput assay with yeast and used this assay to search for chemicals that reduce cell killing as well as DNA deletions from the ASINEX chemical library. Very interestingly, these chemicals turned out to be potent radiation mitigators in mice, when the first dose was applied 24 hours after irradiation. In addition, they significantly reduced the frequency of radiation-induced cancer. In aim 1 the HTS DEL assay in yeast will be used to screen for further chemicals from the libraries and from a targeted library around the current radiation mitigators. In aim 2 we will investigate the effects of the hits from the yeast screen in mammalian cells using a mammalian rearrangement assay. In aim 3 we are proposing to synthesize targeted chemical libraries around those mitigators and select the most potent mitigator from those libraries. We also propose to screen for new mitigators. In aim 4 we will also characterize the mitigators using a variety of assays including a functional genomics assay. Using a functional genomics assay we will determine the genetic control of the mitigation effect in order to obtain novel targets for chemical intervention ofthe mitigation ofthe radiation effect. Such novel mechanistic targets can be used to screen for new compounds that specifically interact with such targets potentially yielding an even more potent mitigator. In aim 5 we will determine radiation mitigation effects ofthe new compounds that we will obtain in vivo. In aim 6 we propose to characterize the mitigators in animals using information will be obtained from projects 2 and 3 with which project 1 will closely interface.